In a general image compressing method, encoding is performed with one picture separated into a plurality of blocks each having a predetermined size. Further, to raise compression efficiency, inter-frame prediction and intra-frame prediction technologies are used that eliminate redundancy between pictures.
An inter-frame prediction-based image encoding method compresses images by removing spatial redundancy between pictures, and a representative example thereof is a motion compensating prediction encoding method.
The motion compensating prediction encoding searches for a region similar to a block being currently encoded in at least one reference picture positioned before and/or behind a picture being currently encoded to thereby generate a motion vector and uses the generated motion vector to perform motion compensation, thereby obtaining a prediction block. Then, the residue of the current block and the prediction block undergoes DCT (Discrete Cosine current block and the prediction block undergoes DCT (Discrete Cosine Transform), quantization, and entropy encoding, and is then transmitted.
In general, a macro block of various sizes, such as 16×16, 8×16, and 8×8 pixels, is used for motion compensating prediction, and a block whose size is 8×8 or 4×4 pixels is used for transform and quantization.
Intra-frame prediction is a method of compressing images by removing spatial redundancy using pixel correlation between blocks in one picture, and this method generates a prediction value of a current block to be encoded from the current block and encoded pixels adjacent to the current block and then compresses the residue of the pixels of the current block and the generated prediction value. In H.264/AVC, the size of a block used for intra-frame prediction is 4×4, 8×8, or 16×16 and each of blocks having a size of 4×4 or 8×8 pixels is subjected to intra-frame prediction using 9 intra-frame prediction modes, and blocks having a size of 16×16 pixels are subjected to intra-frame prediction using 4 intra-frame prediction modes.
As described above, the residue generated by intra-frame prediction undergoes transform and quantization.
However, the conventional transforming methods do not consider characteristics of each block and cannot maximize transforming performance in terms of rate-distortion.